Earth in crossfire on NASA hazardous asteroid map

The orbits of all known Potentially Hazardous Asteroids (PHAs) as of early 2013 are shown here. But none are worrisome threats over the next century.
NASA/JPL-Caltech

If you've been watching the skies for the spectacle of the Perseid meteor shower over the past few days, you've probably thought about some of the larger rocks up above.

A few are downright scary compared to shooting stars. This graphic from NASA shows the orbits of more than 1,400 Potentially Hazardous Asteroids (PHAs), objects measuring at least 460 feet across that come within 4.7 million miles of Earth's orbit.

It looks like a giant Spirograph drawing with Earth near the center, but NASA assures us that these giant boulders of rock and ice are not "a worrisome threat" over the next 100 years.

Of course, 4.7 million miles is really, really far away -- about 20 times the distance to the moon. But not all PHAs have been found. As one looks further into the future, the orbits of known PHAs become less predictable.

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"By continuing to observe and track these asteroids, their orbits can be refined and more precise predictions made of their future close approaches and impact probabilities," NASA's Jet Propulsion Laboratory reported on its Photojournal page.

The recent discovery of the 10,000th near-Earth object (NEO) highlighted that there are a lot of marbles zooming around up there. NEOs are defined as objects approaching the Earth's orbital distance to within some 28 million miles.

Of greatest concern are the roughly 900 near-Earth asteroids that are at least 1 kilometer (0.62 miles) across. Judging by studies of mass-extinction events of the past, an impact by one of these could be a real bummer.